Laser ranging interferometer for GRACE follow-on

The Gravity Recovery and Climate Experiment (GRACE) has produced a wealth of data on Earth gravity, hydrology, glaciology and climate research. To continue that data after the imminent end of the GRACE mission, a follow-on mission is planned to be launched in 2017, as a joint US-German project with a smaller Australian contribution. The satellites will be essentially rebuilt as they were for GRACE using microwave ranging as the primary instrument for measuring changes of the intersatellite distance. In addition and in contrast to the original GRACE mission, a Laser Ranging Interferometer (LRI, previously also called ‘Laser Ranging Instrument’) will be included as a technology demonstrator, which will operate together with the microwave ranging and supply a complimentary set of ranging data with lower noise, and new data on the relative alignment between the spacecraft. The LRI aims for a noise level of 80 nm/ Hz over a distance of up to 270 km and will be the first intersatellite laser ranging interferometer. It shares many technologies with LISA-like gravitational wave observatories. This paper describes the optical architecture including the mechanisms to handle pointing jitter, the main noise sources and their mitigation, and initial laboratory breadboard experiments at AEI Hannover.